1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device having a circuit constituted by a thin film transistor (hereinafter, referred to as TFT). For example, the invention relates to an electro-optic apparatus represented by a liquid crystal display apparatus and a constitution of an electric apparatus mounted with an electro-optic apparatus as a part thereof. Further, the invention relates to a method of fabricating the device. Further, in the specification, a semiconductor device generally indicates an apparatus capable of functioning by utilizing semiconductor properties and an electro-optic apparatus and an electric apparatus, mentioned above, pertain to the category.
2. Description of the Related Art
In recent years, there have been widely carried out researches on a technology in which an amorphous semiconductor film formed above an insulating substrate of glass or the like is subjected to laser annealing to thereby crystallize the amorphous semiconductor and promoting crystalline performance thereof. Silicon is frequently used for the amorphous semiconductor film.
In comparison with a synthesized quartz glass substrate which has frequently be used conventionally, a glass substrate is provided with advantages that the glass substrate is inexpensive and rich in workability and the glass substrate having a large area can easily be fabricated. This is the reason that the above-described researches are carried out. Further, laser is preferably used in crystallizing the amorphous semiconductor since the melting point of the glass substrate is low. Laser can provide high energy only to an amorphous semiconductor film without considerably elevating temperature of a substrate.
Since a crystalline semiconductor is constituted by a number of crystal grains, a film thereof is referred also as a polycrystal semiconductor film. A crystalline semiconductor film formed by being subjected to laser annealing is provided with high mobility and therefore, a thin film transistor (TFT) is formed by using the crystalline semiconductor film and the crystalline semiconductor film is intensively utilized in, for example, a liquid crystal electro-optic apparatus of a monolithic type in which TFTs for driving a pixel and for a drive circuit are fabricated above one sheet of a glass substrate.
Further, there is preferably used a method of carrying out laser annealing by shaping pulse laser beam such as excimer laser to constitute a square spot of several centimeters square or a linear shape having a length equal to or larger than 10 cm at an irradiated face and scanning the laser beam (or moving an irradiated position of laser beam relatively to an irradiated face) since the method is provided with high productivity and is excellent industrially.
Particularly, when linear beam is used, the productivity is high since different from a case of using laser beam in a shape of a spot where scanning in front and rear direction and left and right direction is needed, laser can be irradiated to a total of an irradiated face by scanning the linear beam only in a direction orthogonal to a longitudinal direction thereof. Laser is scanned in the direction orthogonal to the longitudinal direction since the direction is the most efficient scanning direction. Owing to the high productivity, currently, the main stream is being established by using linear beam produced by shaping pulse oscillated excimer laser beam by a pertinent optical system. The technology enables to provide a monolithic type liquid crystal display apparatus formed with TFT (pixel TFT) forming a pixel portion and TFT of a drive circuit provided at a periphery of the pixel portion above one sheet of the substrate.
However, a crystalline semiconductor film fabricated by the laser annealing process, is formed by aggregating a plurality of crystal grains and positions and sizes of the crystal grains are at random. According to TFT fabricated above the glass substrate, for element isolation, the crystalline semiconductor is formed to isolate by patterning in an insular shape. In that case, the crystalline semiconductor cannot be formed by designating positions and sizes of crystal grains. In contrast to inside of a crystal grain, at an interface of the crystal grain (grain boundary), there are numerously present recombination centers and trap centers caused by an amorphous structure or crystal defect. It is known that when a carrier is trapped by the trap center, the potential of the crystal grain is elevated to thereby constitute a barrier against carrier and accordingly, a current transportation characteristic of the carrier is deteriorated. Although the crystalline performance of a semiconductor film at a channel forming region, effects various influence on electric properties of TFT, it is almost impossible to form the channel forming region by a single crystal of semiconductor film by excluding the influence of the grain boundary.
In order to resolve such a problem, according to the laser annealing process, there have been carried out various trials for forming a crystal grain the position of which is controlled and which is provided with a large grain size. Here, an explanation will firstly be given of a procedure of solidifying the semiconductor film after irradiating the semiconductor film with laser beam.
It takes a certain degree of time until solid phase nuclei are generated in a liquid semiconductor film which is completely melted by irradiating the laser beam and the procedure of solidifying the liquid semiconductor film is finished by generating numerous uniform (or nonuniform) nuclei in a completely melted region and growing crystals therefrom. Positions and sizes of crystal grains provided in this case are at random.
Further, when the semiconductor film is not completely melted by irradiating the laser beam and a solid phase semiconductor region partially remains, crystal growth is started from the solid phase semiconductor region immediately after irradiating the laser beam. As has already been mentioned, it takes a certain degree of time to generate nuclei in the completely melted region. Therefore, during a time period until nuclei are generated in the completely melted region, by moving a solid/liquid interface (which designates an interface between the solid phase semiconductor region and the completely melted region and corresponds to a front end of growth of crystal nucleus) constituting the front end of the crystal growth in a direction in parallel with a film face of the semiconductor film (hereinafter, referred to as lateral direction), the crystal grain grows to a length several tens times as much as a film thickness. Such a growth is finished by generating numerous uniform (or nonuniform) nuclei in the completely melted region and growing crystals. Hereinafter, the phenomenon is referred to as super lateral growth.
Also in an amorphous semiconductor film or a crystalline semiconductor film, there is present an energy region of laser beam realizing the super lateral growth. However, the energy region is very narrow, further, a position of providing a crystal grain having a large grain size cannot be controlled. Further, a region other than crystal grains having large grain sizes, is a microcrystal region generating numerous nuclei or an amorphous region.
As has already been explained above, when a temperature gradient in the lateral direction can be controlled (heat flow in the lateral direction can be produced) in the energy region of laser beam for completely melting the semiconductor film, a position of growing a crystal grain and a direction of growing thereof can be controlled. There have been carried out various trials in order to realize the method.
For example, there is a report with regard to a laser annealing process in which a metal film having a high melting point is formed between a substrate and a silicon oxide film of a matrix, an amorphous silicon film is formed above the metal film having the high melting point and laser beam of excimer laser is irradiated from both sides of a surface side of the substrate (defined as a face formed with the film in the specification) and a rear face side (defined as a face on a side opposed to the face formed with the film in the specification), in “R. Ishihara and A. Burtsev: AM- LCD ′98., p 153-p 156, 1998”. The laser beam irradiated from the surface side of the substrate is absorbed by the silicon film and is converted into heat. Meanwhile, the laser beam irradiated from the rear face side of the substrate is absorbed by the metal film having the high melting point and converted into heat to thereby heat the metal film having the high melting point at high temperature. The silicon oxide film between the heated metal film having the high melting point and the silicon film, is operated as a layer for storing heat and therefore, a cooling rate of the molten silicon film can be retarded. It is reported that by forming the metal film having the high melting point at an arbitrary location, there can be provided a crystal grain having a diameter of 6.4 μm at maximum at the arbitrary location.
Further, James S. Im et al of Columbia University show a Sequential Lateral Solidification method (hereinafter, referred to as SLS method) capable of realizing super lateral growth at an arbitrary location. According to the SLS method, crystallization is carried out by shifting a mask in a slit-like shape at every shot by about a distance of carrying out the super lateral growth (about 0.75 μm).
A description has been given by the inventors of a method of carrying out large particle size formation of a crystal grain by providing a stepped difference at a matrix in Japanese Patent Application No. 351060/1999. Here, an explanation will be given of the method.
FIG. 1A shows a first sample when a stepped difference is provided at a matrix insulating film. In the first sample, a silicon oxynitride film (A-type) is formed on a synthesized quartz glass substrate and an amorphous silicon film is formed on the silicon oxynitride film (A-type). A stepped difference is provided at the silicon oxynitride film (A-type) constituting the matrix insulating film to thereby provide a portion having a thin film thickness and a portion having a thick film thickness. In this case, according to the specification, the silicon oxynitride film (A-type) is a silicon oxynitride film having composition ratios of Si=32%, O=59%, N=7% and H=2% and a silicon oxynitride film (B-type) is a silicon oxynitride film having compositions ratios of Si=32%, O=27%, N=24% and H=17%. With regard to the first sample, there is carried out a heat conduction analysis simulation when the amorphous silicon film is crystallized by irradiating laser beam from a surface side of the substrate. The result is shown in FIG. 1B. As conditions used in carrying out the calculation, a wavelength of the laser beam is set to 308 nm, irradiation energy is set to 400 mJ/cm2, a pulse width (a time period of outputting the laser beam) is set to 30 ns and the laser beam is irradiated in vacuum. Table 1 shows other parameters used in the calculation.
The result shown by FIG. 1B is provided since temperature gradient is produced because the matrix insulating film is operated as a heat capacitance. Region B of FIG. 1A is cooled faster than other location since there are both of (1) the matrix insulating film right thereunder and (2) the matrix insulating film present in the lateral direction as locations of escaping heat. Conversely, at region C, temperature is difficult to lower since there is heat escaping from region B to the matrix insulating film right under region C. Therefore, temperature gradient is produced between region B and region C or between region B and region A. By producing the temperature gradient, crystal growth is started from region B having low temperature, a solid/liquid interface is moved to region C or region A having high temperature and accordingly, a crystal grain having a large particle size can be obtained.
That is, a structure used in TFT fabricated above a conventional glass substrate, that is, a structure forming a matrix insulating film on the glass substrate and forming a semiconductor film on the matrix insulating film, stays unchanged, however, in Japanese Patent Application No. 351060/1999, the stepped difference is provided by etching the matrix insulating film at a desired position. When laser beam is irradiated from the surface side of the substrate to the sample, a temperature distribution is produced at an inner portion of the semiconductor film in correspondence with a shape of the stepped difference of the matrix insulating film and a location and a direction of producing lateral growth can be controlled.
It is structurally possible to fabricate TFT of a top gate type with the semiconductor film formed by the method of R. Ishihara et al as an activation layer. However, there is produced a parasitic capacitance by the silicon oxide film provided between the semiconductor film and the metal film having a high melting point and accordingly, power consumption is increased and it is difficult to realize high-speed operation of TFT. Meanwhile, it seems that by constituting a gate electrode by the metal film having a high melting point, the method is effectively applicable to TFT of a bottom gate type or an inverse stagger type. However, in the case of the structure forming the silicon oxide film on the substrate, forming the metal film having high melting point on the silicon oxide film and forming the amorphous silicon film on the metal film having a high melting point, even when a consideration is given thereto by excluding a film thickness of the amorphous silicon film, with regard to film thicknesses of the metal film having a high melting point and the silicon oxide film, film thickness suitable in a crystalline step and film thickness suitable in electric properties of TFT element, do not necessarily coincide with each other and therefore, it is not possible to simultaneously satisfy both of optimum design in the crystalline step and optimum design of the element structure.
Further, when the metal film having a high melting point which is not provided with light transmitting performance, is formed over an entire face of the glass substrate, a transmission type liquid crystal display apparatus cannot be fabricated. A chromium (Cr) film or a titanium (Ti) film used as a metal material having a high melting point, is provided with high internal stress and therefore, there is a high possibility of posing a problem in adherence with the glass substrate. Further, there is a high possibility that influence of the internal stress is effected also to the semiconductor film formed thereabove and is operated as a force causing strain in the formed crystalline semiconductor film.
Meanwhile, in order to control threshold voltage (hereinafter, described as Vth) constituting an important parameter in TFT in a predetermined range, it is necessary to consider a reduction in a charging detect density of a matrix film or a gate insulting film formed by an insulting film in close contact with the activation layer and balance of the internal stress other than electron charging control of the channel forming region. A material including silicon as a constituent element as in the silicon oxide film or the silicon oxynitride film, is suitable to meat such a request. Therefore, there is a concern of deteriorating the balance by providing the metal film having a high melting point between the substrate and the matrix film.
Further, according to the SLS method, fine control by a unit of micrometer is needed in a technology of positioning a mask relative to the substrate and there is constituted an apparatus which is more complicated than an ordinary laser irradiating apparatus. Further, there poses a problem in throughput when the SLS method is used in fabricating TFT applied to a liquid crystal display having a large area region.